Ghosts are produced at a receiver usually because a signal arrives at the receiver through different transmission paths. For example, in a system having a single transmitter, the multipath transmission of a signal may occur because of signal reflection. That is, the receiver receives a transmitted signal and one or more reflections (i.e., ghosts) of the transmitted signal. As another example, the multipath transmission of a signal may occur in a system having plural transmitters that transmit the same signal to a receiver using the same carrier frequency. A network which supports this type of transmission is typically referred to as a single frequency network. Because a ghost results from the multipath transmission of a signal, a ghost is often referred to as multipath interference.
A variety of systems have been devised to deal with the problems caused by ghosts. For example, spread spectrum systems deal very adequately with the problem of a 100% ghost by spreading the transmitted data over a substantial bandwidth. Accordingly, even though a 100% ghost means that some information may be lost, the data can still be recovered because of the high probability that it was spread over frequencies that were not lost because of the ghost. Unfortunately, the data rate associated with spread spectrum systems is typically too low for many applications.
It is also known to transmit data as a data vector. A matched filter in a receiver correlates the received data with reference vectors corresponding to the possible data vectors that can be transmitted. A match indicates the transmitted data. Unfortunately, the data rate typically associated with the use of matched filters is still too low for many applications.
When high data rates are required, equalizers are often used in a receiver in order to reduce ghosts of a main signal. An example of a time domain equalizer is a FIR filter. A FIR filter convolves its response with a received signal in order to recover data and eliminate any ghosts of the data. The coefficients applied by the FIR filter asymptotically decrease toward zero for ghosts that are less than 100%. However, for 100% ghosts, the coefficients applied by the FIR filter do not asymptotically decrease toward zero so that a FIR filter becomes infinitely long if a 100% ghost is to be eliminated, making the FIR filter impractical to eliminate a 100% ghost.
A frequency domain equalizer typically includes a Fast Fourier Transform (FFT) which is applied to the received signal. A multiplier multiplies the frequency domain output of the FFT by a compensation vector. An inverse FFT is applied to the multiplication results in order to transform the multiplication results to the time domain. The compensation vector is arranged to cancel the ghost in the received signal leaving only the main signal. However, information in the received main signal is lost at certain frequencies so that the output of the inverse FFT becomes only an approximation of the transmitted data.
U.S. application Ser. No. 09/158,730 filed Sep. 22, 1998 discloses a vector domain equalizer which effectively eliminates ghosts up to 100% by distributing the transmitted data in both time and frequency so that the vectors are essentially random in the time and frequency domains. Accordingly, in a heavily ghosted channel, all data can be recovered with small noise enhancement, and any enhanced noise that does exist is near white. However, the number of calculations performed by the transform in the receiver to recover the data is large.
U.S. application Ser. No. 09/283,877 filed Apr. 1, 1999 discloses a single path equalizer which effectively eliminates ghosts up to 100% and which uses fewer calculations. This equalizer includes a pre-processor, a finite filter, and a post-processor. The pre-processor multiplies a data input block received from the channel by coefficients b in order to modulate the received main signal and its ghost so that the ghost is less than the received main signal. The finite filter applies coefficients a in order to eliminate the ghost from the multiplication results. The post-processor applies coefficients c to the output of the finite filter in order to reverse the effects of the modulation imposed by the pre-processor. Also, the post-processor applies a window function to the output of the finite filter. This single path equalizer somewhat enhances noise picked up in the channel.
U.S. application Ser. No. 09/425,522 filed Oct. 22, 1999 discloses a dual parallel path equalizer having a pre-processor, a finite filter, and a post-processor in each path. The pre-processors multiply a data input block received from the channel by corresponding coefficients b1 and b2 in order to modulate the received main signal and its ghost so that the ghost is less than the received main signal. The finite filters apply corresponding coefficients a1 and a2 in order to eliminate the ghost from the multiplication results. The postprocessors apply corresponding coefficients c1 and c2 to the outputs of the finite filters in order to reverse the effects of the modulations imposed by the pre-processors. Each of the outputs of the post-processors is a solution to the problem of a ghost. That is, substantially no ghost is present in the output from each of the post-processors. The outputs of the post-processors are added in order to substantially minimize enhancement of noise, thus producing better signal to noise performance as compared to a single path equalizer.
The present invention is directed to an equalizer which uses less hardware than the dual path equalizer describe above, and which also substantially eliminates ghosts up to 100% while at the same time producing good noise performance.